1. Field of the Invention
The present invention concerns an antenna coupling circuit using capacitive coupling.
2. Prior Art
Various means are known for transmitting high-frequency signals through the window glasses of automobiles without holes in said window glasses.
For example, methods in which inductive coupling is accomplished by installing coils facing each other on both sides of an insulator (glass, etc.), and capacitive coupling methods in which capacitive coupling is accomplished by installing electrode conductors facing each other on both sides of such an insulator so that a capacitance is formed, are known. Furthermore, in these methods, tuning to desired frequencies is performed so that transmission can be effectively accomplished.
Examples of the above include U.S. Pat. Nos. 4,089,817; 4,238,799; 4,658,259; 4,785,305; and 4,794,394.
In cases where inductive coupling is accomplished by means of coils in the conventional methods, the coils are ordinarily used with a degree of couplinq K in the vicinity of 1 (the degree of coupling K is the product of the coupling coefficient K and the tuning coefficient Q). In the 900 MHz frequency band used in automobile telephones, the coil diameter required in order to obtain a degree of coupling K of 1 is at least approximately 30 mm due to the thickness of the glass. In this case, furthermore, the coil length must also be approximately 30 mm. Moreover, the dimensions of the case, a metal case in some instances, which accommodates each coil are approximately twice the dimensions mentioned above. Accordingly, in cases where an antenna coupling circuit which uses inductive coupling by means of coils is attached to the window glass of an automobile, the coil parts are large and create an eyesore. These parts are undesirable from an esthetic standpoint, and are also an impediment to proper washing of the vehicle.
Meanwhile, in conventional methods in which signals are transmitted via a capacitance (as described above), an inductance element and a tuning cavity must be combined with the coupling capacitance for tuning purposes. Accordingly, the same problems generated in the case of antenna coupling circuits using inductive coupling by means of coils as described above are also generated here.
Furthermore, methods which transmit signals via capacitance include methods in which two pairs of electrodes are caused to face each other via a glass. In such methods, the certain problems arises in cases where unbalanced lines such as coaxial lines are used as signal lines. In particular, since a polarity exists in the two pairs of electrodes, the direction of attachment of the electrodes that are to form pairs must be the same between the units inside and outside the passenger compartment. In other words, if the direction of attachment of the electrodes that are to form pairs is reversed (i.e., if the electrodes are attached in reverse in structural terms), the prescribed performance cannot be achieved.
In addition to the methods described above, a method is known in which resonance circuits based on distributed constant plane circuits are caused to face each other on both sides of a window glass. However, even in cases where this method is used, the distributed constant circuits must have a large area in order to obtain the desired degree of coupling. As a result, the field of vision through the window glass is obstructed.